French patent application number 85 08 836 describes a technique for exciting a plasma by electron cyclotron resonance. This resonance is obtained for a magnetic field B and an excitation frequency f which are related by the equation: EQU B=2.pi.mf/e
where m and e are the mass and the charge of an electron. For example, at a frequency of 2.45 GHz, a magnetic field of 0.0875 Tesla is required to obtain resonance.
The technique described in the above-mentioned French patent application requires permanent magnets to be used each creating a surface having a constant magnetic field at a strength corresponding to electron cyclotron resonance. Electromagnetic energy is conveyed to the resonance zone by plasma exciters, each constituted by a length of metal wire. Each exciter is disposed inside a plasma reactor in association with permanent magnets which are mounted on the wall of the reactor.
Given that the plasma excited by this technique operates at low pressure (about one-tenth of a pascal, 0.1 pascal), the microwave energy is injected into the reactor via a sealed feedthrough including a connection element which is electrically insulated from the hole it passes through so as to constitute a coaxial type of structure. The connection element is connected on the inside of the enclosure to an exciter, and on the outside of the enclosure to the core of a coaxial cable conveying electromagnetic energy, for example.
When using such an enclosure fitted with a set of exciters, each of the exciters must be fed either from a single microwave source or else from a plurality of independent sources. When a plurality of sources are used, each exciter is associated with at least one microwave source via impedance matching means and possibly via measuring means. When a single source is used, it is necessary to use at least one microwave energy distributor associated with impedance matching means and measuring means. It turns out that using such a technique is not entirely satisfactory in practice.
A first drawback concerns the microwave energy distributors which are generally constituted by coaxial connectors in which the central core penetrates to a greater or lesser extent either into a waveguide or else into a microwave cavity. Except under special circumstances, the positions of the coaxial connectors are complex and need to be calculated by computer. Further, it is difficult to balance the various coaxial connectors relative to one another and numerous successive adjustments are required on the depth of penetration of each connector.
Additional drawbacks of this technique are related to implementing sealed coaxial type feedthroughs for feeding microwave energy into the plasma excitation zone.
Thus, the connections of the connection element of the feedthrough with both the coaxial cable core and the exciter are sources of impedance discontinuity and of heating, thereby giving rise to energy losses.
In addition, the dielectric in the feedthroughs facing the plasma is subjected simultaneously to heating due to the plasma and to contamination due to plasma treatment. The higher microwave power for plasma excitation passing through the feedthrough, the greater the contamination. However, given that microwave energy is absorbed by plasma excitation and falls off with increasing distance from the feedthrough, it appears that the transmission of high power requires coaxial type feedthroughs to be made using difficult and expensive technology.
Further, the application of high microwave powers requires each exciter to be cooled effectively. The use of sealed feedthroughs makes full cooling of the exciters very difficult since cooling is generally obtained by a flow of cooling fluid injected from the free ends of the exciters. In reality, coaxial cables and feedthroughs can be cooled solely by conduction from the exciter. As a result, the acceptable microwave power is limited by the efficiency with which each of the exciters is cooled, and by the power performance of the dielectric used to make the coaxial type feedthroughs.
Another drawback of this plasma excitation technique relates to the way microwave power from the microwave source falls off progressively along the exciter. Microwaves can be propagated only with loss, and the density of the resulting plasma falls off together with the microwave power.
The present invention thus seeks to remedy the drawbacks mentioned above, by providing a device for coupling microwave energy with at least one exciter in such a manner as to obtain better coupling of the microwave energy.
An object of the invention is to provide a device for distribution microwave energy with a set of exciters making it possible to eliminate the use of coaxial type feedthroughs.
Another object of the invention is to provide a distributor device which distributes microwave energy along the entire length of an exciter.